1. Field of the Invention
The present invention relates to a light source, and more particularly though not exclusively, to a light source that emits EUV (extreme ultraviolet) light.
2. Description of the Related Art
In recent semiconductor manufacturing apparatuses, the wavelength of light emitted from a light source has been shortened, and exposure apparatuses using EUV light have attracted attention as next-generation exposure apparatuses.
A laser produced plasma (LPP) light source is typical of EUV light sources used in EUV exposure apparatuses. FIG. 10 is a schematic view of the LPP light source. In this light source, high-intensity laser light (normally having a repetition frequency of several kilohertz) is applied to a target material, such as xenon, thereby producing high-temperature plasma. From the plasma, EUV light having a wavelength of approximately 13 nm is radiated. The radiated EUV light is reflected by a multilayer mirror, and is guided to an illumination optical system of an exposure apparatus. In addition to the above-described xenon, for example, metal is frequently used as the target material.
When plasma is produced in this light source, besides EUV light, an unnecessary substance called debris scatters from the plasma. Debris particles adhere to or collide with the multilayer mirror, and damage of films of the multilayer mirror. This reduces the reflectance of the multilayer mirror.
In order to prevent this deterioration of the mirror, various countermeasures have been devised. For example, Japanese Patent Laid-Open No. 2005-197456 (counterpart: U.S. Pat. No. 6,987,279 B1) discusses using a magnetic field that is applied near plasma so as to prevent debris particles scattering from the plasma from reaching a mirror, as shown in FIG. 11.
In this case, a target is supplied through a nozzle 104 from a target supply unit 103, and laser light is applied from a driving laser 101 to the target. Consequently, plasma 112 is produced, and EUV light 113 is derived from the plasma 112. A magnetic field is applied in the right-left direction of FIG. 11 by passing a current through coils 106 and 107. Consequently, debris particles (charged particles) produced from the plasma 112 move upward or downward while turning around magnetic lines of force, and are guided outside the mirror. In this way, debris particles scattering from the plasma 112 are prevented from reaching the mirror.
However, in the above-described debris-particle eliminating method, it is difficult to capture debris particles scattering from the plasma at high speed unless a considerably strong magnetic field is applied. Conversely, when the strength of the magnetic field is decreased, debris particles easily reach the mirror, and the mirror deteriorates fast.